This invention relates to a noise-eliminating apparatus for a video tape recorder and, more particularly, to a type which can be effectively applied to a circuit for reproducing high frequency signals recorded in a time-division fashion (FM audio signals).
Using the conventional helical scan type video tape recorder (hereafter referred to as the "VTR"), FM audio signals are recorded on an exclusive audio track on a tape by means of an exclusive audio magnetic head. With such an FM-audio signal recording system, however, the frequency property and dynamic range for ensuring a good recording of stereo signals and sound multiplex signals have been found insufficient. This drawback arises from the fact that the tape travels slowly and the continuous audio track is narrow.
For resolution of the above-mentioned difficulties, a system has been proposed which is designed to record both video signals and FM audio signals on the video track. According to this proposed system, the FM audio signals are recorded on the recording tracks by alternately operating first and second video heads. This proposed system fully satisfies the frequency property and dynamic range required for the recording of FM audio signals.
The FM audio signals, time-divisionally recorded by the helical scan system and reproduced by the alternately operating first and second video heads, must be converted into regular, continuous FM audio signals to satisfy the listener.
FIG. 1 shows the conventional circuit for converting first-channel FM audio signals, reproduced from the first video head, and second-channel FM audio signals, reproduced from the second video head, into the regular, continuous FM audio signals. According to said conventional circuit, first-channel FM audio signals (FM 1) are supplied to a first FM demodulator 13 through an input terminal 11, for demodulation, and second-channel FM audio signals (FM 2) are delivered to a second FM demodulator 14 through an input terminal 12. Demodulated output signals from the first and second FM demodulators 13, 14 are respectively carried to the first and second input terminals 15A, 15B of a switch 15. The operation of this switch 15 is controlled in such a manner that the first input terminal 15A is used during the tape scanning period of the first video head, that is, one field, and the second input terminal 15B is used during the tape scanning period of the second video head, that is, one field. As a result, continuous demodulated audio signals are sent forth from an output terminal 16.
According to the above-mentioned conventional circuit, the first and second demodulators 13, 14 demodulate FM audio signals in the duty ratio of 1:1. In other words, the switch 15 selects output demodulated audio signals from the first and second FM demodulators 13, 14 in the duty ratio of 1:1. Consequently, said first and second FM demodulators 13, 14 should have identical properties and functions. Therefore, the first and second FM demodulators 13, 14 are required to be of an expensive, high-quality type. Further, it is necessary to apply an advanced technique in order to ensure the identical properties and functions of both FM demodulators 13 and 14. This matching work consumes a great deal of time.
The first and second input terminals 11, 12 are supplied with high frequency FM audio signals in such a manner that the FM audio signal FM1, for example, is delivered to the first input terminal 11 during one field, and the FM audio signal FM2, for example, is sent forth to the second input terminal 12 during the succeeding field, using this scheme, the rear-end portion of the preceding FM audio signal FM1 should overlap the forward-end portion of the succeeding FM audio signal FM2. The rear-end portion of said FM audio signal FM2, supplied to the second input terminal 12 during said succeeding field should overlap the forward-end portion of said FM audio signal FM1, supplied to the first input terminal 11 during a field following the aforementioned, succeeding field.
Since the FM audio signals FM1, FM2 are originally continuous signals, the phases of both signals FM1, FM2 at the first and secind input terminals 11, 12 should agree with each other. However, it often happens that said phases fail to be identical due to the contraction of a magnetic tape and the irregular rotation of the rotary components (such as the tape-feeding mechanism and rotary cylinder) of the VTR. If the phases of said FM audio signals FM1, FM2 fail to be identical during the period in which the end-portions of said signals overlap each other, then noise arises when these signals are demodulated, e.g. when output signals from the first and second demodulators 13, 14 are converted into continuous signals by the switch 15.
For the resolution of the above-mentioned problem, the Japanese patent disclosure (Kokai) No. 55-80,812 sets forth a system which is characterized in that first and second channel-selecting switches are respectively provided in front of the first and second modulators 13, 14, and said switches are operated at different points of time during the aforesaid period in such that end-portions of the FM audio signals overlap each other. This proposed system indeed has the advantage that even if the phases of the FM audio signals FM1, FM2 are displaced from each other during the aforesaid overlapping period, noises can be suppressed. But said system is still accompanied with the previously described drawback, because, when demodulated output signals from the first and second FM demodulators are converted into a continuous signal, the duty ratio of the demodulated output signals is chosen to be 1:1. In other words, the first and second FM demodulators are required to be of the high-quality, expensive type and also have exactly, identical properties.